Time zones (TZ) represent a temporal assignment to a longitude of the earth. Starting from the 360 degrees of longitude of the earth's circumference, in 1884 these were divided by the 24 hours for one rotation of the earth. The resulting 24 time zones each correspond to 15 degrees of longitude.
According to the given time zone scheme, time changes by one hour after every 15 degrees of longitude. Due to the east-west expansion of large countries such as the USA or Russia, these have several time zones: Thus, taking Hawaii into account, the USA has a total of six time zones, and Russia even extends over eight time zones.
The prime meridian runs through Greenwich, a London borough, and forms the well-known reference time Greenwich Mean Time (GMT). GMT reference time has been replaced by Coordinated Universal Time (UTC), which also has the prime meridian as its reference longitude. The various time zones are linked to this UTC time.
Some important time zones for Europe are Western European Time (WET), which is the reference time zone (UTC) at Greenwich, Central European Time (CET), which is one hour ahead of the reference time zone (UTC+1), and Eastern European Time (EET), which is two hours ahead of Greenwich with UTC+2. The geographical course of the time zones was adapted to the geographical, country-specific and political conditions.